LEIBNIZ: THE DISTINCTION OF MIND AND BODY

[Gr p511] [A VI 4, p1368]

I know that I exist, and nevertheless I am uncertain whether there is any 'body' in the nature of things. Therefore I am not a body.1 This argument is weak, unless it can be shown that extension in itself constitutes a complete substance. For even if this is assumed, it is certainly obvious that thought can never originate from that substance. But this has not yet been shown. And so we know this at least, that thought is not a mode of extension, but we do not know whether or not it is a mode of substance, of which extension is itself another mode.2 For evidently extension and extended matter itself are different in body.
Therefore this is the true method of proving the distinction between mind and body, namely that it is impossible for us to ever know with certainty whether body exists. However I call body everything that is like those things we perceive. But from that it is evident that this is impossible, because it is impossible for us to be able to be made certain about the existence of bodies in any way,3 i.e. it is impossible to ever prove by philosophical arguments whether or not bodies are appearances or substances.

[A VI 4, p1369]

That other men think is no more certain to me than that beasts have feelings.
From the fact that I exist it can be sufficiently understood that I am not alone; for I easily understand that others can be like me, or much more perfect than me, and that I cannot be the cause of their existence.

It is certain that I have an external appearance, but it does not therefore follow that there are just as many substances outside me as seem to be causes of appearances i.e. that there are just as many bodies.
By no means can it be proved by natural reason that divisible or corporeal substances exist.

NOTES:

1. Leibniz is thinking here of Descartes' argument found in Meditations II and Discourse on Method IV.2. Here Leibniz wrote and then deleted: 'But it seems not to be so, because two different modes must be resolved into something common.'3. Here Leibniz wrote and then deleted: 'except through a priori reasoning, from understanding the nature of God'.